The doping of semi-conductor materials is a critical aspect in the production of semi-conductor devices. The electrical properties of semi-conductor substrates such as silicon and germanium, depend upon the diffusion of very carefully controlled minute amounts of suitable impurities into the silicon or germanium crystal structure. Such elements as phosphorus, arsenic, antimony, gallium, boron, aluminum and indium are utilized to create the desired electrical rectification characteristics of the semiconductor material. The type, amount and distribution of impurity in the semi-conductor crystalline structure is absolutely critical to the creation of devices with the desired characteristics.
One of the most popular doping techniques utilized in the industry is the so-called "planar diffusion" method. In this method, the semi-conductor materials, most usually in the form of very thin wafers or disks, perhaps 2, 3 and up to 6 inches in diameter, of the semiconductor material, e.g., silicon, are placed upright in closely spaced slots formed into silica "boats" or support structures. Alternating with the silicon wafers are very thin wafers or disks of the dopant in a suitable chemical form; most usually of a glassy ceramic-like compound, such as boron nitride, aluminum metaphosphate, or the like. Such disks may comprise the doping compound alone; or the doping compound mixed with other inert materials such as Al.sub.2 O.sub.3 or SiO.sub.2, etc. The dopant disks are produced by firing the materials at high temperatures to sinter or even melt them; and then forming compacted masses thereof. The compact masses are then cut and ground to yield wafers of relatively the same dimensions as the semi-conductor wafers.
In any event, the dopant disks are emplaced in the diffusion boats alongside the semi-conductor disks and the entire assemblies are placed into high temperature furnaces. When such furnaces are heated to suitable temperatures, normally in the range of about 700.degree.-1200.degree. C. and in an inert atmosphere such as nitrogen, argon, or helium, the desired chemical species, e.g., B or P, etc. is vaporized from the dopant disk surface. These vapors impinge upon the adjacent semi-conductor disk surfaces where they stick and coat the surface. As the materials remain in the furnace, the desired dopant species gradually diffuses into the interior of the semi-conductor material to thereby alter its electrical properties.
The quantity of dopant and depth of its diffusion into the semi-conductor material can be controlled by proper selection of furnace temperatures and time of residence within the furnace. In any event, such techniques have been extremely successful in producing semi-conductors with the desired electrical properties.
Such planar diffusion techniques are, however, subject to a number of disabilities. These disabilities relate most usually to the physical properties of the dopant disks. Very often the dopant disks are quite brittle and fragile, and the least jarring or bumping will cause them to fracture and break. In other instances the dopant disks are very sensitive to their environment. That is, for some materials, e.g., boron nitrides, the presence of moisture may cause alterations in the crystalline structure whereby dimensional changes occur. These changes may result in cracks or shattering of the disks. Some of the other prior art doping disk materials must be regenerated after one or just a few uses. Most usually the disks must be regenerated in an oxidizing environment to convert these desired elements, e.g. B, into the oxide form in which they are readily vaporized during the diffusion process.
It is obvious that any means to overcome the aforementioned difficulties would present a valuable advance in the art.